Lambert Zone 1 from D-Day 1944 maps

[Howard]

I have some secret 1:12,500 maps of the D-Day landings in 1944, They are marked "The grid on this sheet is Lambert Zone 1. Compiled from Air Photographs on a control provided by existing French Triangulation."

When I reference them in GM 10, then export to Google Earth, I get errors of 100+ metres. I have assumed the datum is NTF (Greenwich) but it looks like a datum error.

A web search has yielded no other clue. Anyone have any ideas?

Thanks

Howard

[global_mapper]

Howard,

That would be my guess for the datum as well, but it's hard to say for sure unless the map indicates something else. Also for a map that old while the data may have a relative accuracy equivalent to a 1:12,00 scale map I don't know that you could guarantee that the absolute accuracy is good to within 100 meters. So the error might just be a systematic error in the map and not a probably of datum setup, so you might just need to shift the map (right-click on it in the Control Center to do this) to account for the error.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[Howard]

Howard,

That would be my guess for the datum as well, but it's hard to say for sure unless the map indicates something else. Also for a map that old while the data may have a relative accuracy equivalent to a 1:12,00 scale map I don't know that you could guarantee that the absolute accuracy is good to within 100 meters.

Mike, an answer in 6 minutes, stunning! My main area of research is maps of the Great War 1914-1918, the official story for those is an absolute accuracy of 20 yards. The same people made the 1944 maps, i.e. the Ordnance Survey, after what was reported to be a cartographic fiasco in the Great War, so I would have hoped 20 yards was achievable just a few years later. They are accurately scanned and well drawn.

Howard

[global_mapper]

Howard,

If the absolute accuracy is that good then it could indeed be a datum issue, although without any further guidance from the map itself I'm not sure what datum might have been used other than NTF. I did check the EPSG database for other old datums covering France and found the Ancienne Triangulation Francaise (Paris) and the Nord de Guerre (Paris) datums, both based on the Plessis 1817 ellipsoid with a Paris meridian, but neither of those are supported in Global Mapper. You could add them as new custom datums and give them a try though.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[Howard]

...I did check the EPSG database for other old datums covering France and found the Ancienne Triangulation Francaise (Paris) and the Nord de Guerre (Paris) datums, both based on the Plessis 1817 ellipsoid with a Paris meridian...

Mike
I have ver 6 of the EPSG database but cannot find any dx, dy, dz values to use to define either of these as a new datum in GM. Is there another way apart from plotting common points then working out the 3 values (or indeed 7 values), a method I have found to be less than satisfactory.

Many thanks

Howard

[global_mapper]

Howard,

I took a look but I can't find any datum transformation parameters defined for those datums either. I think your best bet might just be to stick with NTF and then manually shift the data by right-clicking on it on the Control Center and specifying the distance to shift.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[Howard]

Howard,

I took a look but I can't find any datum transformation parameters defined for those datums either. I think your best bet might just be to stick with NTF and then manually shift the data by right-clicking on it on the Control Center and specifying the distance to shift.

Mike

I have tried shifts in Lat & Long but with mixed success, they fit where they touch, allowing for where features have moved etc.

Most of the maps in the set show no datum information but on one of the D-Day maps there is the legend:-
Lambert Zone 1
Lambert (modified) Conical Orthomorhic
Spheroid Clarke 1880
Origin 55 Gr N. Meridian of Paris
False co-ords of origin 600,000m E, 200,000m N
Scale Factor 8151.6/8152.6
Diff in Long Paris-Greenwich, 2 20' 13.95"

I am sure Lambert (modified) Conical Orthomorhic is the same as Lambert Conical Conformal
Scale factor is equal to 0.999877339,
55 Gr N. Meridian of Paris is 49.5 degrees which puts in mid-way between the 1st and 2nd parallels in GM's Lambert Zone 1 values.

There is also the note "Marginal ticks indicate Nord de Guerre Grid", but they do not coincide with the Lambert grid lines.

Is it possible to work out the dx, dy and dz shifts to WGS84 from this?

I feel that I am so so close to getting this right and that I am missing just one detail, but try as I might, it eludes me!

Howard

[global_mapper]

Howard,

I don't know that you could easily derive datum shift parameters from that limited information. I'm almost wondering if it is the projection definition that is the issue. If you use Lambert Conformal Conic with all of the listed parameters rather than just using Lambert Zone 1 how does it look? You might also try creating a custom datum based on the Clarke 1880 ellipsoid with no datum shift and see if that matches. It's possible they just mapped to the ellipsoid and didn't use a datum.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

[Frank2]

THE manual on WGS84 can be found here. It's not a light read though.
http://earth-info.nga.mil/GandG/publ.../tr8350_2.html